Sunday, February 3, 2013

Further adding to the Raspberry Pi's superfluous power draw are the linear 1.8V and 3.3V regulators (RG1 and RG2 respectively). Currently in place of the 3.3V regulator is the NCP1117/SE8117TA (which in turn feeds the 1.8V regulator in series). Peoplearereportingthat a modification of RG2 to a switch mode variant sees reductions in power consumption between 10 to 25%. I will not be looking to replace the 1.8V regulator as it seems to only offer marginal improvement on top of replacing RG2.

I have chosen to replace this regulator with the RECOM R-783.3-0.5 high efficiency/low ripple switching supply. It offers very high efficiency (91%) at it's minimum voltage limit (4.75V) so at 5V it should offer about the same. It has the following pinout:

Be careful, because it has a slightly different pinout to the NCP1117 on the board (which also uses the tab rather than pin2 for Vout).

Remove the NCP1117 and replace with the RECOM R-783.3-0.5. You will need to work some magic with the pins (be sure to pre-plan as you don't want to have to re-bend the legs as they can break off quite easily). As a tip, I decided to solder Vout to the top of capacitor C11 for convenience, rather than the pad/tab.

Then you can now set the 'ondemand' governor. This will alter the CPU clock speed depending on load.

cpufreq-set -g ondemand

Now reboot. If you don't perform all of this before rebooting (contrary to what's on the site linked above), you will have issues with the pi rebooting because the CPU frequency is too low (as the Broadcom's watchdog time kicks in) or USB devices acting bizarre.